Configuration include directory

Two years ago Magnus submitted a patch to parse all the configuration
files in a directory.  After some discussion I tried to summarize what I
thought the most popular ideas were for moving that forward:

http://archives.postgresql.org/pgsql-hackers/2009-10/msg01452.php
http://archives.postgresql.org/pgsql-hackers/2009-10/msg01631.php

And I've now cleared the bit rot and updated that patch to do what was
discussed.  Main feature set:

-Called by specifying "includedir <directory>".  No changes to the
shipped postgresql.conf yet.
-Takes an input directory name
-If it's not an absolute path, considers that relative to the -D option
(if specified) or PGDATA, the same logic used to locate the
postgresql.conf (unless a full path to it is used)
-Considers all names in that directory that end with *.conf  [Discussion
concluded more flexibility here would be of limited value relative to
how it complicates the implementation]
-Loops over the files found in sorted order by name

The idea here is that it will be easier to write tools that customize
the database configuration if they can just write a new file out, rather
than needing to parse the whole configuration file first.  This would
allow Apache style configuration directories.  My end goal here is to
see all of the work initdb does pushed into a new file included by this
scheme.  People could then expect a functionally empty postgresql.conf
except for an includedir, and the customization would go into 00initdb.
Getting some agreement on that is not necessary for this feature to go
in though; one step at a time.

Here's an example showing this working, including rejection of a
spurious editor backup file in the directory:

$ cat $PGDATA/postgresql.conf | grep ^work_mem
$ tail -n 1 $PGDATA/postgresql.conf
includedir='conf.d'
$ ls $PGDATA/conf.d
00config.conf  00config.conf~
$ cat $PGDATA/conf.d/00config.conf
work_mem=4MB
$ cat $PGDATA/conf.d/00config.conf~
work_mem=2MB
$ psql -c "select name,setting,sourcefile,sourceline from pg_settings
where name='work_mem'"
    name   | setting |
sourcefile                       | sourceline
----------+---------+-------------------------------------------------------+------------
  work_mem | 4096    |
/home/gsmith/pgwork/data/confdir/conf.d/00config.conf |          1

No docs in here yet.  There's one ugly bit of code here I was hoping
(but failed) to avoid.  Right now the server doesn't actually save the
configuration directory anywhere.  Once you leave the initial read in
SelectConfigFiles, that information is gone, and you only have the
configfile.  I decided to make that configdir into a global value.
Seemed easier than trying to pass it around, given how many SIGHUP paths
could lead to this new code.

I can see some potential confusion here in one case.  Let's say someone
specifies a full path to their postgresql.conf file.  They might assume
that the includedir was relative to the directory that file is in.
Let's say configfile is /etc/sysconfig/pgsql/postgresql.conf ; a user
might think that "includedir conf.d" from there would reference
/etc/sysconfig/pgsql/conf.d instead of the $PGDATA/conf.d you actually
get.  Wavering on how to handle that is one reason I didn't try
documenting this yet, the decision I made here may not actually be the
right one.

--
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us